In wireless communication systems performance can generally be improved if the characteristics of a transmission are adapted to the characteristics of the radio channel. Even though there are techniques which perform such adapting without much prior information about the channel, e.g. Incremental Redundancy (IR) used together with some Automatic Repeat Request (ARQ) scheme, the performance can still be improved, if each transmission as such, is better adapted to the channel characteristics.
The term Link Adaptation (LA) is often used when referring to the selection of Modulation and Coding Scheme (MCS) to be used for a certain transmission in an attempt to adapt the characteristics of the transmission to the characteristics of the channel. However, there are many other characteristics of the channel which could be exploited to further improve the performance. One of those is the channels suitability for higher rank transmissions using Spatial Multiplexing (SM), whereby the performance may be improved e.g. in terms of higher data rate and signal strength. It is sometimes possible to utilize this type of transmission in case of Multiple Input Multiple Output (MIMO) setups, where several antennas are present for transmission and reception respectively.
In case of LTE systems, there are different uplink transmission modes defined (refer to 3GPP TS-36.213). Depending on the uplink transmission mode being used, Closed Loop Spatial Multiplexing (CLSM) might be supported as transmission scheme for the Physical Uplink Shared Channel (PUSCH). The term Closed Loop (CL) indicates the need of Channel State Information (CSI) for the transmission scheme to operate efficiently.
In LTE there are basically two ways to get uplink CSI in an eNodeB (evolved Node B, also denoted eNB). First there is the Sounding Reference Symbols (SRS) that are allocated to a wireless device semi-statically and used specifically for channel quality estimation purposes. The other is the Demodulation Reference Symbols (DMRS) that are multiplexed with normal PUSCH transmissions. These are mainly aimed for channel estimation in order to demodulate PUSCH data, but they can also be used for channel quality estimation.
The CSI that has been gained from SRS or DMRS is used to adapt different properties of uplink transmissions. Such properties are transmission rank (the number of spatial layers that are used for the transmission), the Modulation and Coding Scheme (MCS) that are used for each layer, the Precoding Matrix that is used for precoding of the transmission as well as the transmission bandwidth. As an intermediate step to select these parameters, a post-equalizer signal to interference plus noise ratio (SINR) for a layer is calculated. This post-equalizer SINR is calculated given the precoder, rank and allocation size and is used to calculate the MCS for the layer.
In order to be able to calculate the post-equalizer SINR that can be used for link adaptation, a scheduler/link adaptation needs to have access to the complex channel coefficients of the MIMO channel. Since the channel coefficients vary over frequency due to multipath fading several channel matrices needs to be stored to cover the whole system bandwidth. The required number of channel matrices depends on the frequency correlation of the channel. Also, since the multipath fading is unique to each user the channel coefficients also need to be stored per user. All in all, all the required channel coefficients will consume a lot of memory in the eNodeB and does not scale well with the number of users and number of receive antennas.
Further, selecting precoder, rank, allocation size, and MCS typically involves performing an exhaustive search through the possible parameter combinations to see which combination maximizes the throughput for the particular user. This involves a large amount of calculations.
The MIMO link adaptation thus involves complex calculations that are time-consuming and require high processing capacity and also large data storage.